This invention relates to semiconductor transducers in general and more particularly to methods for fabricating single crystal semiconductor devices.
The prior art is replete with patents depicting strain gage sensing configurations or transducers for providing force or pressure measurements. In such transducer arrangements strain or force is measured by a pattern of one or more piezoresistive elements diffused in or secured to a diaphragm fabricated from single or polycrystal silicon. The element pattern, which may be a Wheatstone bridge configuration, provides a change in resistance according to the magnitude of an applied force, which force serves to deflect the force collector upon which the sensing elements are located. The term force collector is used in a generic sense and can include a diaphragm, a beam and so on.
In order to provide for more reliable operation at high temperatures, the prior art fabricated such devices using dielectric isolation techniques. In such configurations, a silicon wafer had fabricated on a surface a dielectric layer of silicon dioxide, a layer of epitaxial silicon was fabricated on the dielectric layer and this silicon layer was polycrystalline. The layer of polycrystalline silicon was employed as a diaphragm to support single crystal sensors which are mechano-photochemically formed from the monocrystalline wafer. For examples of such structures and methods for fabrication, reference can be had to U.S. Pat. No. 3,800,264 entitled High Temperature Transducers and Housings Including Fabrication Methods issued on Mar. 26, 1974 to A. D. Kurt and J. R. Mallon, Jr., and assigned to the assignee herein.
Other patents also form sensor arrangements in layers of polycrystalline silicon, wherein the polycrystalline silicon layer serves as a diaphragm. For example, see U.S. Pat. No. 3,858,150 entitled Polycrystalline Silicon Pressure Sensor issued on Dec. 31, 1974 to Gurtler et al.; U.S. Pat. No. 4,003,127 entitled Polycrystalline Silicon Pressure Transducer issued on Jan. 18, 1977 to Jaffe et al.
In any event, it would be extremely desirable to provide a transducer structure which affords the advantages of dielectric isolation in regard to high temperature operation and overall increased reliability, which structure eliminates polycrystalline silicon as the diaphragm components. Polycrystalline silicon does not possess the mechanical strength associated with single crystal silicon and hence, such devices are more fragile, thus limiting the environments and force levels in which the transducer configurations operate. Moreover polycrystalline does not possess the mechanical stability and freedom from hysterisis of single crystal silicon. Hysterisis is a function of slip between the crystalites of a material. Single crystal silicon is essentially hysterisis free and thus an excellent transducer force collector and sensor structure. The prior art was cognizant of this problem and there was a desire to provide a single crystal silicon substrate operating with a single crystal sensor. However, in order to do so, the prior art bonded a single crystal sensor to a single crystal diaphragm by the use of a glass bonding layer having a lower softening temperature than the insulating layer and the semiconductor and having a matching expansion coefficient. A representative structure is depicted in U.S. Pat. No. 3,922,705 entitled Dielectrically Isolated Integral Silicon Diaphragm or other Semiconductor Product issued on Nov. 25, 1975 to Yerman. The problem with such devices are apparent in that the sensor is separately bonded by means of a glass bond, whose fabrication and characteristics are extremely difficult to control, hence rendering the device extremely difficult to fabricate. Moreover such structures using a relatively thick layer of glass suffer from the limitations of glass as a transducer at elevated temperatures, nonelastic behavior and locked in mechanical stresses due to nonuniform cooling during the fabrication process.
Still other techniques attempt to eliminate silicon and provide a substrate of sapphire or spinel. Such materials act as insulators and permit high temperature operation, but are difficult to manufacture due to the extreme care necessary in matching crystal axes, while such materials as spinel and sapphire are more expensive and more difficult to work with. Additionally, spinel and sapphire are difficult to etch and thus are limited in their ability to be fabricated in the complex and very thin force collector configuration currently used in the manufacture of integrated sensor transducers. A reference depicting such transducer structures is U.S. Pat. No. 4,203,327 entitled Piezoresistive Silicon Strain Sensors and Pressure Transducers Incorporating Them issued on May 20, 1980 to Singh.
It is therefore an object of the present invention to provide an improved semiconductor structure and method for fabrication, which structure is particularly adapted for use and a transducer. The structure employs a single crystal substrate having a single crystal sensor positioned directly on an intermediate dielectric isolation layer.